[kccqzy]

You will not lose a cent here and there just by using float64, for the range of values that banks deal with. For added assurance, just round to the nearest cent after each operation.

[kbolino]

You can round to the nearest .0078125 (1/128) but not to the nearest .01 (1/100) because that number cannot be represented in float64.

To round to the nearest cent, you would need to make cents your units (i.e. the quantity "1 dollar" would be represented as 100 instead of 1.0).

[nurettin]

Within the general context of this discussion, "cannot be represented" is a red herring.

You don't need to have a representation of the exact number 0.1 if you can tolerate errors after the 7th decimal (and it turns out you can). And 0.1+0.1+0.1 does not have to be comparable with 0.3 using operator==. You have an is_close function for that. And accumulation is not an issue because you have rounding and fma for that.

[kbolino]

Ok, but in the context of this thread, it is important. Remember that I'm replying to the statement "For added assurance, just round to the nearest cent after each operation." That is misleading advice and the behavior of floats is just context for why.

First of all, a lot of languages don't include arbitrary rounding in their math libraries at all, only having rounding to integers. Second, in the docs of Python, which does have arbitrary rounding, it specifically says:

    Note: The behavior of round() for floats can be surprising: for example, round(2.675, 2) gives 2.67 instead of the expected 2.68. [...]

Thus I think what I said stands: you cannot round to nearest cent reliably all the time, assuming cent means 0.01. The only rounding you can sort of trust is display rounding because it actually happens after converting to base 10. It's why 2.675 will print as 2.675 in Python even though it won't round as you'd expect. But you'd only do that once at the end of a chain of operations.

In a lot of cases, errors like these don't matter, but the key point is that if the errors don't matter, then they don't need to be "assured" away by dubious rounding either.

[kccqzy]

> First of all, a lot of languages don't include arbitrary rounding in their math libraries at all, only having rounding to integers.

You do the simple, obvious and correct thing: multiply by 100, round to int, convert to double, divide by 100. It does not matter whether the final division by 100 results in an exact value. (You might argue this is inefficient but it's not a correctness problem.)

> for example, round(2.675, 2) gives 2.67 instead of the expected 2.68.

You are not going to execute round(2.675, 2) if you follow my advice of rounding after every operation. Because the error will never reach 0.005. Your argument is moot.

[kbolino]

You can certainly encounter 2.675 as a multiplier, even if you wouldn't have it as a balance.

It doesn't matter that some error starts off way less than 0.005 if rounding then amplifies it. We can find two 2-digit numbers that multiply to get exactly 2.675 in reals, but whose product differs from the float number closest to 2.675 enough to affect rounding:

    abs(      2.14 * 1.25     -       2.675    ) < 0.005
    abs(round(2.14 * 1.25, 2) - round(2.675, 2)) > 0.01

And regarding integer vs fractional rounding, we can see different results for what is nominally the same computation, depending on where the decimal point is:

    abs(round(1.0  * 1.5, 0) - 2.0 ) < 0.1
    abs(round(0.01 * 1.5, 2) - 0.02) > 0.001

Now, I never said that floats were bad. I am only saying that rounding them doesn't work the way one might expect, and shouldn't be done any more than necessary; in many cases, it's not necessary at all.

[nurettin]

floats are also a red herring. Maybe you can continue with someone else.